In the art of fluid flow measurement, vortex or vortex shedding flowmeters have a number of advantages over alternative types of flowmeters such as turbine meters and differential pressure flowmeters. One of these advantages is the simplicity of the in-fluid portion of the flowmeter, which typically includes only a bluff body and a pressure fluctuation sensor. Another is the absence from those flowmeters which use solid-state pressure sensors, of moving parts such as turbine blades and their associated bearings. Together, these advantages are responsible for the widespread use of vortex flowmeters to measure the rate of flow and/or total flow of both liquids and gases.
Vortex flowmeters also, however, have a number of disadvantages which limit their usefulness. One of these disadvantages is the tendency of the sensor output signal to exhibit certain instabilities or unpredictable fluctuations. One such fluctuation, known as "fade," is characterized by a decrease in the magnitude of the sensor output signal that extends over a number of cycles. Another such fluctuation, often referred to as a "missing pulse," is characterized by the absence or near absence of one cycle of the sensor output signal in the midst of succession of normal or near normal cycles. Still another of such fluctuations, known as "jitter," is characterized by unpredictable transient changes in the period of the sensor output signal.
Prior to the present invention, a number of different circuits and methods have been devised in an attempt to deal with the above-described instabilities. Some of these, such as the circuit described in U.S. Pat. No. 4,270,391, attempt to eliminate these instabilities at the input side of the flowmeter by using a phase locked loop circuit. Others attempt to eliminate these instabilities at the output side of the meter by using sample and hold circuits. Still others attempt to eliminate these instabilities by means of circuits, intermediate the input and output sides of the flowmeter, which compensate for signal fluctuations as, for example, by generating pulses for insertion in place of missing pulses.
All known circuits of the above-described type, however, introduce the risk of causing inaccuracies in the flow that they are intended to measure. This is because they all have the effect of using data which is (at least in part) not derived from measured frequency values or of discarding data which is derived from measured frequency values. As a result, to the extent that fluctuations in the sensor output signal represent real fluctuations in the rate of flow, their elimination must be regarded as the introduction of a source of error.